This invention relates to hydraulic braking pressure control valves for use in vehicle braking systems.
Conventionally, in applying brakes on a vehicle a major portion of the load of the vehicle will act on the front wheels and the load acting on the rear wheels will reduce according to the so-called "nose dive phenomenon", thus, the rear wheels will be locked earlier than the front wheels if braking forces applied on the front and rear wheels are equal. Therefore, it is required to reduce the braking pressure applied on the rear wheels as compared with that of the front wheels in accordance with the brake applying force, and there is usually provided a hydraulic braking pressure control valve for controlling braking pressure applied on the rear wheels, such that the braking pressure for the rear wheels will elevate at a reduced rate as compared with the braking pressure supplied to the front wheels.
Recently, there have been provided two braking pressure circuits respectively for the front wheels and rear wheels for securing safety of the vehicle, and in such case it is required to increase the braking force acting on the rear wheels when failure occurs in the pressure circuit for the front wheels and the braking function of front wheels is not available, as compared with a normal operating condition.
One prior art hydraulic braking pressure control valve of the aforementioned kind comprises a piston valve slidably received in a main body of the braking pressure control valve and having a liquid passage therethrough for passing braking liquid therethrough which is supplied to the rear wheels, an actuating piston being slidably received in the main body for receiving the braking pressure of the front wheels to bias the piston valve in one direction, a liquid pressure control spring for biassing the piston valve in a direction opposite to such one direction, and a valve body retained at a predetermined position in the main body and being adapted to engage with a seat formed in the liquid passage to close the passage. Upon depressing a brake pedal the braking pressure in the front wheels moves the piston valve in such one direction and closes the liquid passage at a predetermined pressure level (cut-off pressure) thus controlling the braking pressure for the rear wheels. Thereafter, when liquid pressure is supplied to the pressure control valve the passage repeatingly opens and closes, thus elevating the braking pressure supplied to the rear wheels at a reduced rate as compared with that of the front wheels. And when failure occurs in the pressure circuit for the front wheels, the actuating piston does not act to move the piston valve and thus the liquid passage is maintained to open and the braking pressure of the rear wheels is maintained equal to liquid pressure supplied to the control valve.
However, in a braking pressure control valve of the aforementioned kind, the braking pressure control spring has been arranged to extend between the piston valve and a plug screw-threadingly engaging with the main body, and the valve body has been fixedly retained at a predetermined position by a member associating with the plug. Therefore, it has not been possible to provide load responsive characteristics on the pressure control valve, viz. to change or adjust the braking pressure of the rear wheels by changing the strength of the braking pressure control spring in response to the weight or load distribution of the vehicle. More particularly, when the biassing force of the pressure control spring is changed in accordance with a change in the load by displacing, for example, the plug so as to change the distance between the plug and the piston valve, the valve body will accordingly displace, thus changing the distance between the valve body and the valve seat formed in the liquid passage, and therefore, the desired characteristics of the pressure control valve will not be attained.